Siemens method is known as a method for producing polycrystalline silicon used as a raw material for single-crystal silicon used for production of semiconductors and silicon used for production of solar cells. The Siemens method is a method in which a raw material gas containing chlorosilane is contacted with a heated silicon core wire, and polycrystalline silicon is vapor deposited on the surface of the silicon core wire by a CVD (Chemical Vapor Deposition) method.
When polycrystalline silicon is vapor deposited by the Siemens method, two silicon core wires in the vertical direction and one silicon core wire in the horizontal direction are assembled into a Π-shape (approximately inverted U-shaped) within a reactor of a vapor deposition apparatus, and both ends of the Π-shaped silicon core wire are fixed through a pair of core wire holders to a pair of metal electrodes disposed on a baseplate. The configuration is disclosed in Japanese Patent Publication No. 37-18861 (Patent Literature 1), for example.
Each of the metal electrodes penetrates through the baseplate and an insulator is sandwiched by the metal electrode and the baseplate. The metal electrode is connected to another metal electrode via a wire or to a power supply provided outside the reactor. In order to prevent deposition of polycrystalline silicon during vapor deposition, the metal electrode, the baseplate, and the reactor are cooled using a coolant. The core wire holder fixed to the metal electrode is also cooled by the metal electrode.
While a current is conducted from the metal electrode to heat the silicon core wire in a hydrogen atmosphere to a temperature in the range of not less than 900° C. and not more than 1200° C., a raw material gas such as a mixed gas of trichlorosilane and hydrogen is fed from a gas nozzle into the reactor. Thereby, silicon is vapor deposited on the silicon core wire to form a polycrystalline silicon rod having a desired diameter in a Π-shape.
Unfortunately, during or after such a step of vapor depositing polycrystalline silicon, the polycrystalline silicon rod may be inclined or toppled.
As measures to prevent such inclination or toppling, for example, Japanese Patent Laid-Open No. 2002-234720 (Patent Literature 2) proposes use of a core wire holder having a thermal conductivity more than 145 W/m·K and a coefficient of thermal expansion suitable for that of silicon.